Monitoring the local temperature at the nanoscale is fast becoming a critical task in, at least, three areas of nanoscience, i.e., micro/nano-electronics, integrated photonics and biomedicine, and nano-thermometry is attracting the increasing attention of the scientific community [1,2]. Precise measure of the temperature is extremely challenging, when using conventional techniques, due to insufficient contact between the thermometer and the object/area to be measured [1-4]. This issue could be addressed by using thermometers at the nanoscale, able to give high spatial resolution at sub-micrometer up to nanometer scale, real time temperature mapping and accurate and precise temperature response [1-8]. In the panorama of nanosized temperature probes [5-8], semiconductor nanoparticles or quantum dots (QDs) have emerged as competitive temperature sensor candidates due to their size-tunable optical properties, high quantum yield, good photostability and relatively facile synthesis methods [9-11]. They have shown their potential in nanoscale thermometry due to their temperature-dependent photoluminescence (PL) properties, such as PL intensity, PL peak position and lifetime [12-14]. Characteristic sensitivities in the range 0.1˜0.3 nm K−1 were obtained by applying QDs, in agreement with theoretical calculations [12-14].
Variation in PL intensity and in PL peak position has been investigated in thermometric sensors [7,9,14-17]. However, the PL intensity/peak position of temperature sensors not only depends on the local temperature, but also on many other factors, such as the refractive index of the surrounding matrix/solution, the excitation or detection efficiency, the presence of quenching agents, like oxygen, moisture, etc. . . . [4].
Sensitivity of the PL temperature sensors to the above-mentioned changes in the local environment is difficult to be controlled in complex systems such as living cells or micro-devices, leading to inaccurate temperature measurements [15,18,19]. An appealing alternative is the use of double emitting systems, in which two emission bands at different wavelengths are simultaneously monitored. Accordingly, dual emission temperature sensors have been explored to overcome such problems [4].
Dual-emission QDs-based temperature sensors exhibit double luminescence from two excited states in the same QD. One or both of the two PL intensities change, when the temperature is varied. The temperature is typically measured from a ratio of the intensity of the two emission channels, instead of their absolute PL intensities, as in single-emission materials, endowing self-calibration of the system and increasing the robustness and reliability of intensity-based spectroscopy thermometry [13,20]. Vlaskin et al. [20] first reported a dual-emission temperature sensor by using Mn2+ doped Zn1-xMnxSe/ZnCdSe core/shell QDs, based on a dual-emission from two excited states in thermal equilibrium. In a QD-based temperature sensor, several factors may affect the accuracy of the measurement, such as photobleaching and photoblinking under continuous illumination.
There is still a need for robust, accurate and precise temperature sensors at the nanoscale. Also, there is a need for such temperature sensors that operate in a wide temperature range.